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Materials Chemistry and Physics 112 (2008) 977–983

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

reparation and characterization of copper metal nanoparticles usingendrimers as protectively colloids

ei Jina,b, Shi-Ping Yanga,∗, Qi-Wei Tiana, Hui-Xia Wua, Ying-Jun Caia

Department of Chemistry, Shanghai Normal University, Shanghai 200234, PR ChinaDepartment of Chemical and Engineering, Shanghai Petrochemical Academy, Shanghai 201512, PR China

r t i c l e i n f o

rticle history:eceived 11 July 2007eceived in revised form 30 May 2008ccepted 29 June 2008

a b s t r a c t

In this paper, crystal copper nanoparticle clusters, prepared by reduction of CuSO4 in the presence of den-drimers with a trimesyl core (G3–G6), are characterized by Fourier transformation IR (FT-IR), UV spectraand transmission electron microscopy (TEM). The results show that the particle size of the Cu nanoparti-

eywords:endrimersopperanoparticles

cles is considerably affected by the generation of the dendrimers as well as the dendrimers concentration.When the concentration ratios of Cu2+ to the G3–G6 dendrimers are 8:1, the average diameters of theparticles obtained were 5.6, 4.8, 3.9 and 3.4 nm, respectively. When the concentration ratios of Cu2+ todendrimers are 2:1, 4:1, 8:1 and 16:1, the average diameters of the particles are 4.6, 5.1, 5.6 and 6.9 nmfor G3, 3.9, 4.3, 4.8 and 5.6 nm for G4, 3.0, 3.4, 3.9 and 4.2 nm for G5 and 2.6, 3.1, 3.4 and 3.8 nm for G6,respectively. The data from the High-resolution transmission electron microscopy (HR-TEM) and electron

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. Introduction

Recently, metal nanoparticles have been intensively preparednd characterized for their extensive applications in catalysis, elec-rooptical devices, electronic devices, imaging materials [1], and son. Fabrication of nanoparticles has become one of the importantopics in nanotechnology. Accordingly, for that purpose it is verymportant to be able to control the particle size, shape and size dis-ribution of metal nanoparticles. In the wet method, various metalanoparticles have been prepared in the presence of polymers orurfactants as a protective colloid [2].

Since the first synthesis of poly(propyleneimine) dendrimersn 1978 [3], dendrimers have attracted much attention because ofheir well-defined structures and chemical versatility [4]. Gener-lly, dendrimers of low generation tend to exist in relatively openorms, while higher generation dendrimers take a spherical three-imensional structure, which is very different from linear polymersdopting random-coil structure. Dendrimers might provide reac-ion sites including their interior or periphery. Accordingly, it is

xpected that nanoparticles prepared in the presence of den-rimers in solution are affected by the generation of dendrimersnd show different behavior from those prepared using conven-ional linear polymers. In fact, Crook’s group [5–7] and Esumi’s

∗ Corresponding author. Tel.: +86 21 64322343; fax: +86 21 64322511.E-mail address: shipingy@shnu.edu.cn (S.-P. Yang).

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254-0584/$ – see front matter © 2008 Published by Elsevier B.V.oi:10.1016/j.matchemphys.2008.06.073

rticles belonged to simple cube crystal structure.© 2008 Published by Elsevier B.V.

roup [8–10] have characterized the metal nanoparticles obtainedn the presence of poly(amidoamine) dendrimers (PAMAM) witharious surface groups, such as amino, carboxyl or hydroxyl andave found various applications of such nanosized materials.

In this work, the Cu nanoparticles are formed in the presence of3–G6 PAMAM dendrimers with a trimesyl core (DT) (Scheme 1).

t has been found that they are good templates and stabilizers forhe synthesis of Cu nanoparticles. Our approach relies on the coor-ination between Cu ions and nitrogens of dendrimers. Reducinghe Cu ions with NaBH4 produced colloid solutions are stabilized byhe dendrimers. UV–vis absorption spectra, transmission electron

icroscopy (TEM) are employed to characterize the formation ofhe Cu nanoparticles and the morphology of the Cu nanoparticles.

. Materials and methods

.1. Materials

Dendrimers with a trimesyl core are synthesized according to Ref. [11]. Thetructures of DT dendrimers are given in Scheme 1. Metal salts are purchasedrom Shanghai FUNA New Material Ltd. Deionized water is used in all experi-

ents. Sodium borohydride and the other reagents are purchased from the Chemicaleagent Company of Shanghai (China) and are used without further purification. Allhe chemicals are of analytical grade.

.2. Preparation of dendrimer–metal nanoparticles

Take the preparation of Cu nanoparticles using G3 dendrimers as template forxample. A solution of CuSO4 (10.0 ml, 4.0 mmol L−1) is dropped into a solution of3 dendrimers (10.0 ml, 0.5 mmol L−1), and the mixed solutions are stirred for 1.5 h.resh solution of sodium borohydride (10.0 ml, 0.15 mol L−1) is dropped into the

978 L. Jin et al. / Materials Chemistry and Physics 112 (2008) 977–983

Scheme 1. The structure of G3.0–G6.0 DT dendrimers.

L. Jin et al. / Materials Chemistry and Physics 112 (2008) 977–983 979

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olutions above quickly. Then the solutions were stirred vigorously for 40 min. Thexperiments were performed in the air.

. Characterization

The nanoparticles obtained are characterized by TEM (JEM-100, JEOL). The TEM specimens are prepared by dripping a fewrops of a liquid sample onto a carbon-coated copper grid and then

llowing the drops to dry in air. The mean particle diameter is cal-ulated by counting 100 particles from the enlarged photographs.V–vis spectra of the solutions before and after the reduction of

he dendrimers–metal salts are measured by a UV spectropho-ometer (UV-8500). Fourier transformation IR (FT-IR) spectra for

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endrimers and metal particles are recorded on a Nicolet Avatar70 FT-IR spectrometer.

. Results and discussion

.1. Synthesis and characterization of copper nanoparticles

In this work, we employ a technique including a two steps

equence to prepare copper nanoparticles. First, copper ions areoordinated to dendrimers. Then, chemical reduction of Cu (II) withxcess NaBH4 results in intradendrimer copper clusters. FT-IR spec-ra are applied to prove the coordination reaction of the copper ionsith the internal amido groups of PAMAM. Fig. 1 shows the FT-IR

980 L. Jin et al. / Materials Chemistry and Physics 112 (2008) 977–983

Fig. 1. FT-IR spectra of G3 dendrimer alone and G3 dendrimer–Cu2+.Fig. 4. UV–vis spectra of the G3–Cu2+ solution at different concentration ratios.

Fig. 2. Changes in solution color of G3–Cu2+ before (a) and after reduction (b).

Fig. 3. UV–vis spectra of the solution before and after the reduction of the G3–Cu2+ to G6–Cu2+ (a–d). [CuSO4] = 4 mmol L−1, [G3–G6] = 0.5 mmol L−1, [NaBH4] = 0.15 mol L−1.

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L. Jin et al. / Materials Chemist

pectra from the water solution of the dendrimers (G3), the mix-ure of dendrimers and the copper ions (G3–Cu2+). As it can be seen,he G3 dendrimers spectrum exhibits two broad peaks centered ata 1648 and 1543 cm−1, assigned to the C O stretching (amide I)nd N–H bending/C–N stretching (amide II) vibrations of the den-

rimers [12,13], respectively. When the copper ions are added intohe solution of the dendrimers (G3), it is found that amide II peak ata 1543 cm−1 weaken obviously, and amide I peak at ca 1648 cm−1

hifts to 1637 cm−1. These results strongly indicate the coordina-

ig. 5. Transmission electron microscopy (TEM) and particle size distribution of copperd). [CuSO4] = 4 mmol L−1, [G3–G6] = 0.5 mmol L−1, [NaBH4] = 0.15 mol L−1.

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Physics 112 (2008) 977–983 981

ion reaction between the copper ions and the nitrogen or oxygentoms from amide groups when copper ions are introduced intohe solution of the dendrimers [14].

During the process of the second step, the immediate change in

nanoparticles obtained in the presence of dendrimer G3 (a), G4 (b), G5 (c) and G6

ant evidence for the reduction of Cu (II) to Cu (0) (Fig. 2). Goldenrown can exit for an hour in the air, suggesting Cu nanoparti-les can be stable in the air. Meanwhile, the intense alteration ofV–vis spectra before and after reduction of the dendrimers–Cu2+

9 ry and Physics 112 (2008) 977–983

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82 L. Jin et al. / Materials Chemist

olution (Fig. 3) is another attestation for the reduction reaction.n the presence of G3–Cu2+, there is a broad absorption band cen-ered at ca 535 nm, which corresponds to the d–d transition foru2+ in a tetragonally distorted octahedral or square-planar lig-nd field [15]. Increasing the concentration ratio of Cu2+ to G3endrimers, the d–d transition is increased (Fig. 4). When the con-entration ratio of Cu2+ to G3 dendrimers is increased from 2:1o 8:1, the d–d transition is increased and did not shift. Whenncreased the concentration ratio of Cu2+ to G3 dendrimers to 16:1,he d–d transition red shift a lot, which suggests each G3 dendrimeran strongly absorb up to about 8–16 copper ions. Because a G3endrimer contains 42 amines and Cu2+ ion is quadridentate com-ound, it is tempting to conclude that each Cu2+ is coordinated tobout 4 amine groups [15]. In addition, a strong band centered ata 310 nm, can be assigned to a ligand-to-metal change transferLMCT) transition [16]. But when after reduction, these absorbanceands disappear and are replaced with a monotonically increasingpectrum of nearly exponential slope towards shorter wavelengths.he exponential shape is characteristic of a band-like electronictructure, suggesting that the reduced Cu does not exist as iso-ated atoms, but rather as copper clusters [17]. Meanwhile, aftereduction, an observable Mie plasmon resonance band at ca 567 nmFig. 2(a)) [17–19] appears, which indicates that the Cu clustersrepared in dendrimers G3 are comparatively larger in diameter.his large size is a consequence of agglomeration of Cu parti-les adsorbed to unprotect dendrimers exterior [20,21], becauseendrimers G3 is not a confined system. Other higher genera-ion dendrimers–Cu2+ have the similar UV–vis spectra as G3–Cu2+

efore and after reduction, but there are no absorption peak aris-ng from Mie plasmon resonance (around 570 nm), indicating thatu clusters prepared in higher generation dendrimers are smallerhan the Mieonset particle diameter [18,22,23]. Plasmon resonanceannot be detected for very small metal clusters because the peaks flattened due to the large imaginary dielectric constant of such

aterials [17].Dendrimers from G3 to G6 are all used as template for the syn-

hesis of copper nanoparticles. TEM micrographs and the particleize distribution of the copper nanoparticles obtained using dif-erent generation of DT dendrimers as templates are shown inig. 5(a)–(d), respectively.

The data from Fig. 5 indicate that all of these copper nanopar-icles shape are roughly spherical. The average diameter of thearticles in lower generation is larger than that of in higher gener-tion (G3, 5.6 nm; G4, 4.8 nm; G5, 3.9 nm and G6, 3.4 nm). Fig. 5(a)hows that the Cu particles are big and not dispersed well becauseower generation dendrimers (G3) are open and flat in shape. G3endrimers do not hold confine structures, so they do not con-ain interior void spaces to encapsulate particles. In this way, thearticles aggregated easily. The results from Fig. 5(a)–(d) indicatehat these Cu particles are getting more and more small, regular,nd well dispersed with the increase of dendrimers generationumber. Because higher generation dendrimers take a sphericalhree-dimensional structure [1] and copper particles are encapsu-ated in their interior sites, the particles have no chance to aggregatend become small and regular. Especially for G6, we can see thatany small particles compose a bigger sphere, which reflects indi-

ectly the shape of the dendrimer. That is to say, up to generation, the dendrimers become spherical structure. It also suggests thathe nanoparticles are enveloped in dendrimer’s interior void sites,o the aggregation of the particles is inhibited.

Not only the generation of the dendrimers influences the size ofhe copper nanoparticles as described above, but also the concen-ration ratio of Cu2+ to dendrimers influences the nanoparticle size.n order to obtain the relationship between the particle size andhe concentration ratio of Cu2+ to dendrimers, the average diame-

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ig. 6. Changes in average particle diameter of copper nanoparticles with ratio ofCu2+]/[dendrimer].

er of the copper particles is plotted against the concentration ratiof Cu2+ to dendrimers for G3–G6 dendrimers. As shown in Fig. 6he average diameter increases with the increase of concentrationatio of Cu2+ to dendrimers. For example, when the concentrationatio of Cu2+ to dendrimers is 2:1, the average diameter of cop-er nanoparticles is 4.6 nm for G3, 3.9 nm for G4, 3.0 nm for G5nd 2.6 nm for G6, respectively. Increasing the concentration ratiof Cu2+ to dendrimers from 4:1, 8:1 to 16:1, the average diameterf copper nanoparticles increased to 5.1, 4.3, 3.4 and 3.1 nm, 5.6,.8, 3.9 and 3.4 nm and 6.9, 5.6, 4.2 and 3.8 nm for G4, G5 and G6,espectively. Compared with other linear polymers [1], only a verymall amount of the dendrimers is required to obtain nanometer-ized copper particles. Therefore, the dendrimers can function asvery effective protective template for the preparation of copperarticles.

The result from aqueous solution shows that the copperanoparticles was also affected by pH values. With the pH valueecrease, the copper nanoparticles become aggregate and not wellispersed. When pH values decrease, the amine groups of theendrimers were protoned and Cu2+ ions cannot be well coor-inated by nitrogen atoms from dendrimers and Cu ions cannotnter into cavity of the dendrimers, which result in that the den-rimers do not act as templates at all. In our experiments, pH valuesere 9.

.2. High-resolution transmission electron microscopy (HR-TEM)nd electron diffraction

HR-TEM measurement is performed on the stabilized copperanoparticles using dendrimers as templates. Fig. 7(a) shows thathe copper particle is 3.8 nm and it is spherical. The lattice fringes ofhe particle suggest that the copper nanoparticles are crystal struc-ures. The growth orientation of the crystal is vertical to the latticeringes, which reflects the crystalline order of the copper nanopar-icles [24]. The measured interplanar spacing is ca 0.9 Å. Fig. 7(b) ishe electron diffraction of the crystal (the same sample as Fig. 7(a)).

les are spathic structures. The ratios of these Debye rings’ radiusre nearly 2:3:6, which corresponds to the surface of 1 1 0, 1 1 1,1 1, respectively, as shown in Fig. 7(b). According to the litera-

ures, this kind of crystal belongs to simple cube crystal structure25].

L. Jin et al. / Materials Chemistry and Physics 112 (2008) 977–983 983

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. Conclusion

The copper crystal nanoparticles are obtained and characterizedsing dendrimers as templates. In lower generation dendrimers,he diameter of the Cu cluster nanoparticles obtained are large,hile in higher generation, that of the nanoparticles is small due

o the structure of the dendrimers. In addition, the diameter ofhe nanoparticles is also related to the concentration ratio of theopper ions to the dendrimers and decreases with the decreasingoncentration ratio of Cu2+ to dendrimers.

cknowledgements

This work is supported by the Key Project from the Science andechnology Foundation of Shanghai (No. 065212050), the Rising-tar Program of Shanghai (04qmx1444), the Shanghai Leadingcademic Discipline Project (T0402) and the Key Subject of Edu-ation Ministry of China.

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